The present invention relates generally to implantable medical devices and, more particularly, to acquiring and processing multiple signals representative of several physiologic parameters associated with cardiac and respiratory functions.
Various approaches have been developed for detecting certain physiologic parameters using trans-thoracic impedance measuring techniques. Such known approaches generally involve measuring changes in impedance between endocardial, subcutaneous or intrathoracic lead electrodes owing to the beating action of the heart, respiration or other physiologic activities. Measuring two physiologic parameters, for example, requires a expenditure of battery power in the form of current supplied during each impedance measurement in order to separately measure each of the physiologic parameters of interest. It is understood that a considerable percentage of battery life may be consumed for purposes of making necessary and investigatory trans-thoracic impedance measurements using conventional approaches.
One particular disadvantage of conventional trans-thoracic impedance measuring approaches results from inaccuracies that arise when discriminating between physiologic signals detected by impedance sensing techniques. Such known impedance sensing techniques are highly dependent on the geometrical path length and tissue inhomogeneities between source and sense electrodes. Measured physiologic signals are often highly modulated by the change in geometrical distance or electrical properties of tissue between two sense electrodes. This characteristic, which conventional approaches have attempted to address, has been found to reduce the accuracy of certain physiologic parameter measurements.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading the present specification, there is a need in the art for improved trans-thoracic impedance measuring techniques for measuring a variety of physiologic parameters of interest. There exists a further need for such techniques that provide for reduced, rather than increased, battery power consumption. The present invention fulfills these and other needs.
The present invention is directed to a system and method implemented with an implantable medical device (IMD). In accordance with a broad characterization of the present invention, a high frequency source current signal is propagated through all or a portion of the body. A multiplicity of high frequency voltages are concurrently detected at a multiplicity of body locations in response to the high frequency source current signal. Each of the multiplicity of detected high frequency voltages is associated with a particular physiologic parameter. The locations of the detected high frequency voltages are judiciously selected to enhance detection of a particular physiologic parameter of interest.
Physiologic voltages are extracted from the detected high frequency voltages, typically through use of a demodulation technique. A multiplicity of physiologic output signals are produced using the extracted physiologic voltages. The physiologic output signals may be used for a variety of purposes, including monitoring of numerous cardiac and respiratory function parameters, for example.
According to one embodiment of the present invention, parameters associated with cardiac function and/or respiratory function are derived using a single drive, dual sense impedance technique of the present invention. A multiplicity of sense voltages are concurrently acquired in response to a single source current event. In an alternative embodiment, a multiplicity of sense voltages are concurrently acquired in response to a continuous source current. The sense voltages are developed at particular locations of the heart and chest cavity that provide for enhanced detection of a particular physiologic components of interest. Although cardiac function and respiratory function represent two such physiologic components of interest, other physiologic functions associated with the pulmonary system or other systems of the human body may be advantageously evaluated using techniques of the present invention.
In accordance with one embodiment, an implantable medical device includes a housing and a header electrode, also referred to as a header indifferent electrode. It is understood that the housing may further include a can electrode rather than, or in addition to, a header electrode (e.g., a common header/can electrode). A high frequency source current signal is propagated through at least a portion of a heart. The source current signal has a frequency greater than a frequency of a pacing current signal producible by the IMD. The source current preferably has a frequency that avoids disturbance with a pacing function of the IMD (e.g., source current does not directly stimulate the heart).
In response to the source current signal, a first voltage is sensed between two portions of a first region of the heart substantially concurrently with sensing a second voltage between one of the IMD housing, header, and can electrode and one of the two portions of the first region of the heart. The first voltage, in the embodiment, is associated with a cardiac function and the second voltage is indicative of a respiratory function.
The source current signal may take several forms. For example, the source current signal may take the form of a single cycle current pulse or a multiple cycle current pulse. The source current signal may also be a continuous current signal. Further, the source current signal may take the form of a monophasic current pulse or have a polyphasic character, such as in the case of a biphasic current pulse.
By way of further example, the source current signal may be a multiple cycle current signal having a duration of between 1 and 10 cycles. The source current signal may also be a current signal having an amperage of between about 30 micro-amps and about 2 milli-amps. The source current signal typically has a frequency of between about 5 KHz and about 100 KHz.
The first and second voltages are typically high frequency voltages having respective frequencies equal to the frequency of the current signal. The first and second voltages are demodulated to develop first and second signals respectively associated with cardiac and respiratory functions. For example, the cardiac function of interest may be cardiac contraction or cardiac relaxation. Other parameters of cardiac function that may be indicated by the first signal include a flow of blood, volume of blood, or pressure of blood associated with the cardiac function. For example, the first signal may be indicative of a hemodynamic signal, such as a hemodynamic maximum sensor rate (HMSR) signal.
The second signal may, for example, be indicative of a rate, pattern, or depth of breathing associated with the respiratory function. By way of example, the second signal may be indicative of a ventilation signal, such as a minute ventilation signal.
In accordance with another embodiment of the present invention, a source current signal is propagated through at least a portion of a heart. In response to the source current signal, a first voltage is sensed between two portions of a first region of the heart. Concurrently with sensing of the first voltage and in response to the source current signal, a second voltage is sensed between one of the IMD housing, header, and can electrode and one of the two portions of the first region of the heart portion. A first signal indicative of a cardiac function is developed using the first voltage. A second signal indicative of a respiratory function is developed using the second voltage.
The source current signal may be introduced at one of the two portions of the first region of the heart portion. The first voltage may be a differential signal that is not referenced to the IMD housing. The first voltage may, for example, be sensed between two portions of a ventricle of the heart.
The first voltage may be developed between two electrodes of a common lead. Each electrode is coupled to a respective one of the two portions of the first region of the heart, typically in the right ventricle region. The first voltage is thus developed in the ventricle of the heart. The second voltage is sensed between one of the two electrodes in the ventricle and one of the IMD housing, header, and can electrode.
In accordance with another embodiment, an implantable medical device that implements a single drive, multiple sense methodology of the present invention includes a housing, a header electrode and/or a can electrode, and a current generator that produces a source current signal. A lead is coupled to the current generator. The lead includes a first electrode and a second electrode. The first and second electrodes are situated respectively at two portions of a first region of a heart. For example, the first and second electrodes may be ventricular electrodes. The source current signal is delivered to the heart via the first or second electrode.
A detector unit is coupled to the lead. The detector unit detects, in response to the source current signal, a first voltage between the first and second electrodes substantially concurrently with detecting a second voltage between one of the IMD housing, header, and can electrode and one of the first and second electrodes. The detector unit uses the first voltage to develop a first signal indicative of a cardiac function and uses the second voltage to develop a second signal indicative of a respiratory function.
In one configuration, the detector unit includes a first detector coupled to the lead that detects the first voltage and uses the first voltage to develop the first signal indicative of the cardiac function. The detector unit further includes a second detector coupled to the lead that detects the second voltage and uses the second voltage to develop the second signal indicative of the respiratory function.
The source current signal, as discussed previously, is typically a high frequency current signal having a frequency that avoids disturbance with a pacing function of the IMD. The source current signal, for example, is a high frequency current signal having a frequency greater than a frequency of a pacing current signal producible by the IMD. In this embodiment, the first and second voltages are high frequency voltages having respective frequencies equal to the frequency of the current signal. The detector unit includes a demodulator that demodulates the first and second voltages to respectively develop the first and second signals.
The lead may be a bipolar lead. The first electrode may be a tip electrode and the second electrode may be a ring electrode. In this arrangement, the first voltage is detected between the tip and ring electrodes of the lead. The second voltage is detected between one of the IMD housing, header, and can electrode and one of the tip and ring electrodes.
The above summary of the present invention is not intended to describe each embodiment or every implementation of the present invention. Advantages and attainments, together with a more complete understanding of the invention, will become apparent and appreciated by referring to the following detailed description and claims taken in conjunction with the accompanying drawings.